The present invention relates to the art of protection circuitry for a power amplifier and, more particularly, for RF power amplifiers of the type employed in AM radio broadcasting.
The U.S. Pat. to H. I. Swanson, No. 4,580,111, discloses an amplitude modulator which generates an amplitude modulated carrier signal by selectively turning on and off a plurality of RF amplifiers in a digital manner to produce amplitude modulation. Each of the RF amplifiers includes a plurality of switching transistors, each of which may take the form of a MOSFET transistor, connected together in a bridge circuit. The output terminals of each bridge circuit are connected across a primary winding of a transformer having a secondary winding. The secondary windings of all of the power amplifiers are connected together in series for combining the amplified RF signals with the combined RF signal being supplied to a load, such as an antenna. If one or more of the power amplifiers is on and delivering power to the load, the other amplifiers that are not on will have the same current flowing in their secondary windings and this is reflected in the primary windings. The bottom transistors of such a bridge arrangement are reversed biased until it is decided to turn on the RF power amplifier. However, even though the bottom transistors are reverse biased, current will flow in the top transistors as a result of the current flowing in the primary winding. The top transistors will provide a low impedance path for this current flow in both directions. Thus, for current flow in one direction, the impedance is low because the transistor gate is turned positive during this time. A current path for the other direction of current flow is provided by the MOSFET body drain diode. If the bottom transistors are turned on by removing the reverse bias during the time that current is flowing in the body drain diode of the associated top transistor, a large stress will be generated in the top transistor by forcing its body drain diode to recover very rapidly. Also, high peak current will flow in the bottom transistor. Most MOSFETS will fail under this type of stress.
It has been known in the prior art that failure of such transistors may be avoided by carefully adjusting the timing at which the reverse bias is removed to turn on the lower transistors such that this takes place at exactly the zero crossing of the RF signal supplied to the bridge circuit. However, if the turn on is too early, the transistors in one half of the bridge amplifier will be stressed. If the timing is too late, the transistors in the other half of the bridge amplifier will be stressed.